All-optical fiber-based communication network designs typically include hierarchies of linked local subnetworks and long haul routes, some of which may simultaneously operate at the same optical wavelength because of their physical separation. Wavelength division multiplexing is employed by such a network for maximizing signal capacity of the various network routes and subnetworks. The network signal capacity may be further increased by use of local subnetworks that are reconfigurable to accommodate any one of a range of optical frequencies at a given time, depending on the most efficient frequency configuration of the network at that time. Such reconfigurability further results in improved flexibility of routing protocols used to direct signals across the network and thereby improves routing efficiency.
Fundamental building blocks for such a multi-frequency optical communications network are efficient optical frequency shifters. In addition to shifting of frequencies for routing to subnetwork links, frequency shifters are required for phase conjugation operations employed over long haul routes, as well as for various types of optical processing, e.g., optical logical components for performing Boolean functions.
A variety of optical frequency conversion techniques have been demonstrated, but each of these exhibits one or more performance limitations. For example, one historically common approach employs electrical detection of an optical frequency followed by frequency remodulation of a separate laser. This electrical detection technique is inherently limited by the speed of electronics, which cannot yet meet the terahertz capabilities of optical fiber, and importantly, cannot easily accommodate multiple wavelengths simultaneously.
In another known frequency conversion technique, gain saturation of a semiconductor amplifier is employed as an optical frequency modulator. Like the electrical detection technique, such an amplifier gain saturation technique has limitations in that its frequency conversion is modulation-dependent, its speed is limited by electrical carrier effects, and in a conventional configuration, it cannot accommodate multiple wavelengths simultaneously.
The simplicity and availability of dispersion shifted fiber has enabled several fiber-based frequency shifter techniques which utilize four wave mixing of an input signal and a pump signal in a length of fiber to produce a conjugate signal to the input signal, the conjugate signal, by definition, being at a frequency shifted from that of the input signal. Four wave mixing is a well-known nonlinear optical phenomenon in which two or more signals interact, due to a nonlinear medium, e.g. a medium exhibiting a nonlinear index of refraction, in which they signals are mixed, to generate conjugate signal frequencies as a function of the input signal frequencies.
In one such scheme, shown by Inoue in "Polarization-insensitive wavelength conversion using fiber four-wave mixing with two orthogonal pumps at different frequencies," in OFC '94, February 1994, pp. 251-252, two pump lasers are polarization multiplexed and then combined with an optical signal whose frequency is to be shifted. The pump-signal combination is then input to a length of fiber; four wave mixing of the pumps and the signal take place due to the nonlinear index of refraction of the fiber as the fiber length is traversed to produce a conjugate to the input signal having a frequency determined by the pump and input signal frequencies. At the fiber output, optical filters are required to separate the desired frequency-shifted conjugate signal from the pump signals and the input signal.
In an alternative scheme, shown by Hasegawa et al. in "Polarization independent frequency conversion by fiber four-wave mixing with a polarization diversity technique," in IEEE Photon. Tech. Lett., v. 5, n. 8, August 1993, pp. 947-948, an optical signal to be frequency shifted is first combined with a pump laser signal and then the combined signal is injected into a loop of dispersion-shifted fiber for four wave mixing of the signal components. A polarizing beam splitter (PBS) is used to inject the combined signal into the loop such that one polarization component of the combination signal travels clockwise around the loop and the other polarization component of the combination signal travels counterclockwise around the loop. Hasegawa relies on pre-combination of pump and input signals so that the PSB identically polarizes the pump and input signals as they are injected in combination into each arm of the loop. The input signal, pump signal, and conjugate signal generated by four wave mixing in the loop all exit the PBS at a common exit port, thereby requiring optical filtering to separate out the desired conjugate signal.
Morioka, in "Polarisation-independent 100 Gbit/s all-optical demultiplexer using four-wave mixing in a polarisation-maintaining fibre loop," Elect. Lett., v. 30, n. 7, March. 1994, pp. 591-592, show a similar scheme that uses a loop of polarization-maintaining dispersion-shifted fiber to produce four wave mixing of an input signal and a pump signal. Like Hasegawa, Morioka first combines the input signal and pump signal and then injects the combined signal to the fiber loop through one port of a PBS. Here, however, the pump, input signal, and generated conjugate signal together exit the PBS at the same port through which they were injected. A Faraday circulator is employed to isolate the input signal from the combination output signal, which must be filtered to eliminate the pump signal and produce the desired frequency-shifted conjugate signal.
All of these fiber-based conversion schemes are limited because in each case, the output of the frequency shifter must be filtered to eliminate the pump signal from the output channel of the shifter. Additionally, these schemes require filtering of the pump signal before it is combined with an input signal to suppress any pump amplified spontaneous emission (ASE) associated with the pump. As a practical matter, pump ASE can be expected to leak through the system and so must additionally be filtered out of the output channel. Any pump or pump ASE coupled into the output channel can result in a severe decrease of as much as 20 dB in the signal-to-noise ratio of the frequency-shifted signal, and further results in interference of the pump signal with other channels that are wavelength division multiplexed with the frequency-shifted output channel signal.
The optical pump and pump ASE filtering required by the fiber-based conversion schemes described above inherently limits the overall performance of these conversion schemes. Firstly, the complexity of the conversion systems are increased by the pumps because each additional filter component in the conversion system must be precisely aligned and tuned on-the-fly to accommodate a specific frequency to be converted at a given time. Additionally, due to inherent nonideal filtering by the optical filters, the overall conversion efficiency of the system is correspondingly reduced by the filters.
But perhaps most critically, optical filters available at this time require a finite amount of time for on-the-fly filter frequency tuning, and so significantly reduce the speed of conversion of these systems. As a result, the speed of an optical logic operation or the speed with which an optical subnetwork using the conversion schemes may be reconfigured to accommodate a network-commanded frequency shift is accordingly reduced. The design of an efficient, high-speed frequency shifter having a high signal-to-noise ratio has thus remained a daunting optical processing challenge met with only suboptimal results.